Liquid Mirror Telescopes Set For Magnetic Upgrade 64
KentuckyFC writes "Liquid mirror telescopes start life as a puddle of mercury in a bowl. Set the bowl spinning and the mercury spreads out in a thin film giving the surface an almost perfect mirror finish. But these telescopes have two important limitations. First, they can only point straight up since tilting the mirror spills the mercury. And second, they cannot be made adaptive to correct for any blurring introduced by the Earth's atmosphere. But liquid mirror telescopes look set for an upgrade thanks to the work of a group of Canadian researchers. Their technique is to change the shape of the liquid mirror using powerful electromagnets. They use a ferromagnetic fluid of iron nanoparticles in oil instead of mercury which is too dense to be easily manipulated in this way. The work is just proof of principle at this stage but the idea is to use magnets to correct for the usual range of optical aberrations that telescopes have to deal with (abstract). And also to allow a liquid telescope to be tilted by using oil that is much more viscous than mercury and correcting any periodic deformation in the fluid that tilting might cause."
Re:Power outage (Score:3, Informative)
Re:Alternately (Score:3, Informative)
Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.
Too dense? (Score:5, Informative)
"They use a ferromagnetic fluid of iron nanoparticles in oil instead of mercury which is too dense to be easily manipulated in this way."
Well, that and the fact that a ferrofluid (== ferromagnetic or antiferromagnetic, ir depends) is a little easier to influence with magnetic fields than an weak diamagnet like mercury...
Re:Interesting design (Score:5, Informative)
When you talk about space, everything changes. If the ferrofluid has a volatile base liquid, it will all evaporate/boil away in the vacuum (and make a heck of a mess of the rest of the telescope). I couldn't tell from the ferrofluid manufacturer web site, but the material doesn't make any claims about vacuum compatibility (the stuff is used to make seals but those look to be hermetic and not vacuum seals).
The other problem with space applications and these thin deformable mirrors is whether there is any savings in making a mirror out of them over glass. If the weight of all the actuators, actuator support structures, electronics to run the actuators and the control system, etc. weigh more than a proper piece of glass of comparable diameter, then you're better off going with a nice stable piece of glass.
As an aside, I'm not so sure it makes it easier to build larger interferometric arrays. Everything behind the primary telescope mirrors stays the same and you are only talking about how much gain you get building these mirrors over glass. For interferometric arrays what is important is the "filled" area vs the area of the effective diameter, and unless you're talking about these mirrors being an order of magnitude larger (and much cheaper than the glass ones), I'm not so sure it impacts your "filled" vs "unfilled" area ratio.
Re:Smooth Magnetic Field (Score:4, Informative)
Is it a real parabola? (Score:3, Informative)
That question immediately came to mind, since as wild guess I would expect something more like a catenary. At http://www.math.iupui.edu/m261vis/LMirror/mirrorproof.html [iupui.edu] they show that it really is a parabola.
Re:Interesting design (Score:4, Informative)
With small electromagnets, it is possible to generate fields of this magnitude, on this scale. The magnetic field inside a solenoid is
B = mu_0 * (N/L) * I
where mu_0 permeability 4*PI e-7
N - Number of turns
L - Length of solenoid
I - Current in solenoid
Typical Values of N = 5000, L = 1 cm, I = 0.5 A, B = 314 mT at the center (so ~ 150 - 200 mT at the edge).
Re:Smooth Magnetic Field (Score:5, Informative)
The ferromagnetic liquid will always try to achieve an equilibrium point between gravity, surface tension and the surrounding magnetic field. Gravity and surface tension will make it try and remain flat. As a magnetic field is continuous, it should be possible to have a large number of small but powerful magnets to make the liquid adopt whatever position is desired.
There are a good few videos on youtube: Magnetic sculpture [youtube.com]
Re:Is it a real parabola? (Score:3, Informative)
Re:Smooth Magnetic Field (Score:3, Informative)
Re:I love ferrofluid (Score:3, Informative)
Iron is very common in ink such as the classical iron gall [wikipedia.org] and black tattoo ink [about.com].
Re:Interesting design (Score:1, Informative)
A big glass mirror can be put together from smaller hexagonal glass mirror pieces. Some of the larger surface bound telescopes are set up that way too. So the launch vehicle diameter is not a hard limit.